The NYPD doesn’t know why somebody replaced the Brooklyn Bridge’s American flags with white ones, and they don’t believe it’s terror-related — but they’re not taking it lightly.

“Needless to say, no matter what the motive was, it is a matter of concern,” Police Commissioner Bill Bratton told reporters Tuesday. “I am not particularly happy about the event.”

Construction workers noticed the flags had been swapped out around 5:30 a.m. Tuesday, Deputy Commissioner John Miller said, and alerted police. Emergency services cops climbed up the towers to inspect the scene.

“What they found were two flags, one on each tower, that were approximately 20-feet-by-11-feet, that actually appear to be American flags, with stars and stripes, that had been bleached white,” Mr. Miller said.

If somebody wasn’t trying to make a statement with the flags, it’s been lost on the NYPD.

“At this time, it appears it has no particular nexus to terrorism or even politics. This may be somebody’s art project, or it may be an attempt at making some kind of statement — but at this point it’s not clear what that statement is,” Mr. Miller said.

Surveillance footage shows a group of about 5 people walking over the bridge shortly after 5 a.m., who are of “particular interest” to the department. Footage later shows the lights on each tower briefly flickering and going out, with a 13-minute gap between when that happens on each tower.

Police found large aluminum pans that had been affixed over the lights with zip ties, to provide the cover of darkness for the bizarre capture-the-flag operation.

The climbers were able to get around a locked gate, climb the towers and knew the size of the light bulbs, Mr. Miller pointed out.

“There’s some indication of some good deal of pre-operation planning, perhaps some indication that they have experience climbing in construction or in bridgework, or that they have actually been up there before looking at the dimensions,” Mr. Miller said.

Mr. Bratton said it’s a good reminder of the ubiquitous policing phrase: “If you see something, say something.”

“We are so dependent on the public seeing something out of the ordinary and helping out — this is a reinforcement of that need,” Mr. Bratton said.